1. Field of the invention
The present invention relates to a television synchronous receiver for use in a television receiver and a VTR video tuner.
2. Description of the Prior Art
So-called electronic tuners having tuning circuits each comprised of a varactor diode and an inductor are widely used in recent years in television receivers and VTR video tuners. The electronic tuners are advantageous in that they have no contacts and hence no contact failures, and are capable of providing many functions such as remote control because of their electronic controllability. However, the electronic tuners fail to have characteristics as originally designed since the varactor diodes have irregular characteristics and require the inductors for tuning, and are difficult to manufacture without adjustment and on an automated basis.
The inventor has already invented a television synchronous receiver employing a Costas loop to replace receivers having a tuning circuit composed of a varactor diode and an inductor.
The prior television synchronous receiver invented by the inventor will hereinafter be described with reference to the drawings. FIG. 1 of the accompanying drawings is a block diagram of the prior art television synchronous receiver. The television synchronous receiver comprises a high-frequency input unit 1, a first synchronous detector 2, a second synchronous detector 3, a first low-pass filter 4, a second low-pass filter 5, a first signal amplifier 6, a second signal amplifier 7, a phase comparator 8, a third low-pass filter 9, a voltage adder 10, a voltage-controlled oscillator 11, a 90.degree. phase shifter 12, a channel selection voltage generator 13, and a video signal filter 14.
Operation of the television synchronous receiver thus constructed will hereinafter be described. A video carrier signal v.sub.i (t) of a desired channel received by the high-frequency input unit 1 is subjected to vestigial-sideband modulation and expressed by: ##EQU1## where Re indicates a real part of the formula within the braces { }, I(t) is an in-phase component of the modulated carrier and contains a video signal, Q(t) is a quadrature component of the modulated carrier, .omega..sub.i is the angular frequency of the video carrier, and .phi..sub.i is the phase of the video carrier. The video carrier signal v.sub.i (t) is applied via the high-frequency input unit 1 to one input terminal of the first synchronous detector 2.
An output from the voltage-controlled oscillator 11 is expressed by: EQU v.sub.o (t)=A.sub.o cos (.omega..sub.o t+.phi..sub.o) (2)
and, when this output is applied to the other input terminal of the first synchronous detector 2 which comprises a voltage multiplier, the first synchronous detector 2 produces an output v.sub.I (t) expressed as follows: ##EQU2## When the output from the voltage-controlled oscillator is synchronized with the video carrier, .omega..sub.o =.omega..sub.i, and the output v.sub.I (t) is expressed by: ##EQU3## By removing the 2.omega..sub.i signal with the low-pass filter 4, the output v.sub.I (t) becomes: ##EQU4## where .phi. is .phi..sub.i -.phi..sub.o which is the phase difference between the video carrier and the output from the voltage-controlled oscillator. If .phi.=0, then ##EQU5## Therefore, the signal component I(t) in phase with the video carrier is obtained as the detected output. However, no quadrature component is detected. The detected output is applied through the low-pass filter 4 and the signal amplifier 6 to the video signal filter 14.
When a television signal is received with a superheterodyne receiver system, the overall baseband frequency characteristics can be regarded as flat due to the characteristics of an intermediate-frequency amplifier having a Nyquist slope. However, when a television signal is received with a synchronous receiver system, the overall baseband frequency has characteristics as shown in FIG. 2(a), in which the voltage gain in the low frequency range is twice that in the high frequency range. With the prior art arrangement as illustrated in FIG. 1, the video signal filter 14 has frequency characteristics as shown in FIG. 2(b) to correct the overall baseband frequency characteristics.
The prior television synchronous receiver described above with respect to its construction and operation utilizes a Costas loop which is one type of the synchronous carrier recovery system. Therefore, even when the received television signal is weak, the output from a local oscillator can be brought into synchronism with the received television signal. The above prior art arrangement however has had a problem in that a carrier chrominance signal in a channel lower than and adjacent to a desired channel to be received, a partial luminance signal, and a carrier sound signal are mixed as a disturbance signal into a baseband video signal.
More specifically, a disturbance signal is mixed which will be described with reference to FIG. 3. A carrier television signal is composed of signals having the frequency relationship as shown in FIG. 3(a), the desired channel being shown on the righthand side and the lower adjacent channel on the lefthand side. A television signal in the desired channel is subjected to synchronous detection in the synchronous detector 2 so that the signal is converted into a baseband video signal, a carrier chrominance signal, and a carrier sound signal as shown in FIG. 3(b). Likewise, a television signal in the lower adjacent channel is converted by the synchronous detector 2 into an adjacent carrier video signal, an adjacent carrier chrominance signal, and an adjacent carrier sound signal as shown in FIG. 3(c). The frequency component as indicated by the hatched area in FIG. 3(c) is removed when the output from the synchronous detector 2 passes through the low-pass filter 4, the removed component contains most of the energy of the adjacent carrier video signal. However, the other components shown in FIG. 3(c), mainly the adjacent carrier chrominance signal and the adjacent carrier sound signal are mixed into the baseband video signal illustrated in FIG. 3(a).